Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of materials coated upon a substrate and employing a cover affixed to the substrate around the periphery of the OLED device. The thin-film layers of materials can include, for example, organic materials, electrodes, conductors, and silicon electronic components as are known and taught in the OLED art. The cover includes a cavity to avoid contacting the cover to the thin-film layers of materials when the cover is affixed to the substrate.
OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic electroluminescent (EL) element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EML) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EML layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EML can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
Referring to FIG. 2, an OLED device as taught in the prior art includes a substrate 10 on which are formed thin-film electronic components 20, for example conductors, thin-film transistors, and capacitors in an active-matrix device or conductors in a passive-matrix device. The thin-film electronic components 20 can cover a portion of the substrate 10 or the entire substrate 10, depending on the OLED device design. Over the substrate 10 are formed one or more first electrode(s) 14. One or more layers of organic materials 16 are formed over the first electrode(s) 14, at least one layer of which is light emitting. One or more second electrode(s) 18 are formed over the layers of organic materials 16. A cover 12 with a cavity forming a gap 32 to avoid contacting the thin-film layers 14, 16, 18 is affixed to the substrate 10. In some designs, it is proposed to fill the gap 32 with a curable polymer or resin material to provide additional rigidity. The second electrode(s) 18 may be continuous over the surface of the OLED. Upon the application of a voltage across the first and second electrodes 14 and 18 provided by the thin-film electronic components 20, a current can flow through the organic material layers 16 to cause one of the organic layers to emit light 50a through the cover 12 (if it, any material in the gap 32, and the second electrode 18 are transparent) or to emit light 50b through the substrate 10 (if it and the first electrode 14 are transparent). If light is emitted through the substrate 10 it is a bottom-emitter OLED and the thin-film electronic components 20 may occlude some of the light 50b emitted or may limit the emission area to the area 26 between the thin-film electronic components 20, thereby reducing the aperture ratio of the OLED device. If light is emitted through the cover 12 the OLED device is a top-emitter and the thin-film electronic components 20 do not necessarily occlude the emitted light. The arrangement used in FIG. 2 is typically a bottom emitter configuration with a thick, highly conductive, reflective electrode 18 and suffers from a reduced aperture ratio. Referring to FIG. 3, a top-emitter configuration can locate a first electrode 14 partially over the thin-film electronic components 20 thereby increasing the amount of light-emitting area 26. Since, in this top-emitter case, the first electrode 14 does not transmit light, it can be thick, opaque, and highly conductive. However, the second electrode must then be transparent.
In commercial practice, the substrate and cover have comprised 0.7 mm thick glass, for example as employed in the Eastman Kodak Company LS633 digital camera. For relatively small devices, for example less than five inches in diagonal, the use of a cavity in a cover 12 is an effective means of providing relatively rigid protection to the thin-film layers of materials 16. However, for very large devices, the substrate 10 or cover 12, even when composed of rigid materials like glass and employing materials in the gap 32, can bend slightly and cause the inside of the cover 12 or gap materials to contact or press upon the thin-film layers of materials 16, possibly damaging them and reducing the utility of the OLED device.
It is known to employ spacer elements to separate thin sheets of materials. For example, U.S. Pat. No. 6,259,204 B1 entitled “Organic electroluminescent device” describes the use of spacers to control the height of a sealing sheet above a substrate. Such an application does not, however, provide protection to thin-film layers of materials in an OLED device. US20040027327 A1 entitled “Components and methods for use in electro-optic displays” published 20040212 describes the use of spacer beads introduced between a backplane and a front plane laminate to prevent extrusion of a sealing material when laminating the backplane to the front plane of a flexible display. However, in this design, any thin-film layers of materials are not protected when the cover is stressed. Moreover, the sealing material will reduce the transparency of the device and requires additional manufacturing steps.
U.S. Pat. No. 6,821,828 B2 entitled “Method of manufacturing a semiconductor device” granted 20041123 describes an organic resin film such as an acrylic resin film patterned to form columnar spacers in desired positions in order to keep two substrates apart. The gap between the substrates is filled with liquid crystal materials. The columnar spacers may be replaced by spherical spacers sprayed onto the entire surface of the substrate. Similarly, U.S. Pat. No. 6,559,594 entitled “Light Emitting Device” describes the use of a resin spacer formed on the inside of the cover of an EL device. However, such a resin spacer may de-gas and requires expensive photolithographic processing and may interfere with the employment of color filters. Moreover, columnar spacers are formed lithographically and require complex processing steps and expensive materials. Moreover, this design is applied to liquid crystal devices and does not provide protection to thin-film structures deposited on a substrate. Additionally, rigid non-compressible spacers will transfer applied pressure directly to underlying layers, potentially damaging them.
U.S. Pat. No. 6,551,440 B2 entitled “Method of manufacturing color electroluminescent display apparatus and method of bonding light-transmitting substrates” granted 20030422. In this invention, a spacer of a predetermined grain diameter is interposed between substrates to maintain a predetermined distance between the substrates. When a sealing resin deposited between the substrates spreads, surface tension draws the substrates together. The substrates are prevented from being in absolute contact by interposing the spacer between the substrates, so that the resin can smoothly be spread between the substrates. This design does not provide protection to thin-film structures deposited on a substrate The use of cured resins is also optically problematic for top-emitting OLED devices. As is well known, a significant portion of the light emitted by an OLED may be trapped in the OLED layers, substrate, or cover. By filling the gap with a resin or polymer material, this problem may be exacerbated.
There is a need therefore for an improved OLED device structure that improves the robustness and performance of the device and reduces manufacturing costs.